DIY: Replace the Coolant Level Sensor on a 1991 BMW E91 to Seal a Leak

Spotting the Leak

When a 1991 BMW E91 starts to show signs of a coolant leak, the first clue is often a puddle of fluid under the car. The video shows a small, but persistent leak that is not huge, yet it is enough to lower the coolant level. The coolant appears slightly lower than normal, and the color of the fluid is not the expected blue of fresh G48 coolant. The narrator notes,

"I highly doubt that it's blue when it's down by the water pump and thermostat." – Narrator

This suggests that the fluid may be old or a different substance altogether, prompting a deeper inspection of the cooling system.

Removing the Coolant Level Sensor

The coolant level sensor is a small, threaded component that sits at the bottom of the coolant tank. To access it, the video walks through a series of steps: first, the radiator hoses—both the upper and lower—are disconnected. The upper hose is the one that feeds the tank, while the lower hose connects to the bottom of the tank. Two 10‑mm bolts secure the tank to the engine block; these are removed to free the tank for inspection.

Once the tank is detached, a battery filler is used to drain the remaining coolant. The narrator emphasizes that the correct coolant for the E91 is blue G48, but the fluid seen under the car was a different color, indicating that the coolant had either aged or been replaced with an incompatible type. The video shows the tank being tipped upside down to allow the old fluid to drain into a catch can, a process that can be messy but is essential for a thorough clean.

Draining and Cleaning the Tank

With the tank empty, the next step is to inspect the interior for debris or corrosion. The narrator uses a Harbor Freight pick and a small catch can to scrape away any buildup from the bottom of the tank. The process is described as “working it” to remove the old coolant and any sludge that may have accumulated. The video also mentions that the coolant level sensor itself was removed and replaced, as it had been inserted incorrectly and was likely contributing to the leak.

After cleaning, the tank is reassembled. The new sensor is inserted into the bottom of the tank, and the bolts are tightened. The narrator notes the importance of not forcing the sensor into place, as it should snap in with a gentle twist. Once the sensor is seated, the radiator hoses are reconnected, and the tank is secured back onto the engine block.

Reassembly and Bleeding the System

Reassembling the cooling system is only half the battle; the air trapped inside must be purged. The video demonstrates a simple bleeding procedure: the coolant reservoir cap is opened, and the bleeder screw is turned about four rotations to allow air to escape. A battery charger set to 12 V is used to keep the car’s battery topped up during the process, as the coolant system can draw significant current while filling.

The narrator fills the tank with a gallon of premixed coolant, noting that this amount is sufficient to start the bleed cycle. The coolant level is monitored until it reaches the top of the reservoir. After the initial fill, the bleeder screw is closed, and the system is allowed to run for about twelve minutes with the heater and AC on full. The video shows the coolant bubbling and the heater output increasing, indicating that air pockets have been removed.

Once the system is bled, the car is started and the heater is cranked up to confirm that the coolant is circulating properly. The narrator checks for any remaining leaks under the car and notes that the coolant level remains stable. The final check involves turning the engine off, letting it cool, and inspecting the coolant tank for any signs of seepage. The video ends with a confirmation that the coolant level sensor replacement and bleed have resolved the leak, and the car is ready to drive again.

Key Takeaways

Replacing a coolant level sensor on a 1991 BMW E91 is a manageable task for a DIY enthusiast, provided the steps are followed carefully. First, identify the leak and confirm the coolant type. Second, remove the sensor and clean the tank thoroughly. Third, reassemble the system, ensuring all bolts and hoses are tightened properly. Finally, bleed the system to eliminate air pockets and verify that the coolant level remains steady. By following these steps, you can restore the cooling system’s integrity and prevent future overheating or damage.